CALORIMETRIC STUDY OF THE RELATIONSHIP BETWEEN MOLECULAR-STRUCTURE AND LIQUID-CRYSTALLINITY OF ROD-LIKE MESOGENS .1. HEAT-CAPACITIES AND PHASE-TRANSITIONS OF 4'-PROPYLBIPHENYL-4-CARBONITRILE AND TRANS, TRANS-4'-PROPYLBICYCLOHEXYL-4-CARBONITRILE

The heat capacities of the rod-like compounds 4'-propylbiphenyl-4-carbonitrile (3-BBCN) and trans,trans-4'-propylbicyclohexyl-4-carbonitrile (3-CCCN) have been measured with an adiabatic calorimeter between 12 and 383 K. 3-BBCN is not mesogenic and melts into an isotropic liquid at 338.77K, whereas 3-CCCN is mesogenic and its melting and clearing points are 330.73 K and 353.80 K, respectively. The enthalpy and entropy of fusion of 3-BBCN are 22.7 kJ mol-1 and 67.0 J K-1 mol-1, respectively, while those of 3-CCCN are 27.0 kJ mol-1 and 81.7 J K-1 mol-1, respectively. The enthalpy and entropy gain due to the nematic-isotropic transition of 3-CCCN are 1.8 kJ mol-1 and 5.0 J K-1 mol-1. 3-CCCN shows a mesomorphic transition from a smectic to the nematic state at 329.62 K, which occurs as a metastable state, its transition enthalpy and entropy are 5.4 kJ mol-1 and 16.4 J K-1 mol-1, respectively. The temperature dependence of the molar entropies of both compounds shows that molecular arrangement in the crystal is more ordered in 3-CCCN than in 3-BBCN. This fact may be related to the stability of mesophases. Finally, Eidenschink's theoretical model for the nematic-isotropic transition has been applied to 3-CCCN. As far as the present mesogen is concerned, the transition enthalpy estimated according to this model agrees well with the observed value.